The present invention relates to surfaces reflecting electromagnetic waves, such as antenna reflectors, electromagnetic shielding, waveguides, etc., as well as to a process for producing these surfaces.
Surfaces reflecting electromagnetic wavesxe2x80x94hereafter called reflective surfacesxe2x80x94are already known which are produced from sheets of metal shaped, for example by drawing, so as to give them a self-supporting structure. However, such surfaces have a large mass so that they are generally limited in size. In addition, because of their mass, they cannot be mounted on board spacecraft.
Therefore, in order to remedy these mass and size-limitation drawbacks it has already been proposed to produce reflective surfaces by metallizing, using any known method (spraying, electroplating, vacuum deposition, conductive paint, etc.), supports made of a composite comprising carbon fibers and a cured-resin matrix. It is thus possible to obtain reflective surfaces of acceptable mass and of the desired size. However, these reflective surfaces have drawbacks. Firstly, it is found that the rectilinear portions of the carbon fibers of said supports introduce an undesirable parasitic polarization in the electromagnetic radiation reflected by said surfaces. This is due to the fact that the carbon fibers partly reflect the incident electromagnetic radiation while the cured resin of the matrix, lying between said fibers, is relatively transparent to said radiation.
In addition, local parasitic electrical discharges may occur between the facing ends of broken carbon fibersxe2x80x94these fibers being conductivexe2x80x94thereby generating interference in said reflected radiation.
Finally, the metallization of said composite supports generally has a surface finish so smooth that the thermal radiation received by such a reflector is concentrated onto the focus of the latter. Thus, when the source of the reflector lies at the focus it is necessary to thermally protect said source, for example by covering the active surface of the reflector with a diffusing paint.
The object of the present invention is to remedy the drawbacks of reflective surfaces based on a composite support, while still making it possible to obtain reflective surfaces of comparable lightness.
For this purpose, according to the invention, the rigid surface reflecting electromagnetic waves, especially for antenna reflectors, electromagnetic shielding and waveguides, is noteworthy in that it consists of an interlacement of electrically conducting wires which consist, on the surface, of a stable metal diffusion alloy ensuring that said wires are fastened together and that said surface is rigidified.
Thus, by virtue of the present invention, the carbon fibers and their drawbacks (parasitic polarization and fracture-induced discharges) are eliminated. Moreover, since in said interlacement the conducting wires are crossed, forming microfacets, the surface, when it is present in the form of an antenna reflector, no longer focuses the thermal energy just onto the focus; on the contrary, this thermal energy passes via a focal spot. As a result, the source is exposed to a smaller heat flux and the thermal protection of the source and of the reflector may be less complex. It is no longer necessary to cover the active surface of the reflector with a diffusing paint, thereby avoiding the distortions generated by the latter.
To obtain the rigid reflective surface according to the present invention, it is possible:
to produce a flexible interlacement of electrically conducting wires, the surface of which is metallic and coated with a filler metal, said filler metal having a melting point below that of the surface metal of said wires, and both said filler metal and said surface metal being able to diffuse mutually one into the other when they are heated to a temperature at least equal to the melting point of said filler metal in order to form a stable metal diffusion alloy, the melting temperature of which is above the melting point of said filler metal and increases toward the melting point of said surface metal with the intensity of said diffusion;
to shape said flexible interlacement to the shape desired for said rigid surface reflecting electromagnetic waves; and
to raise the temperature of said flexible interlacement, thus shaped, beyond the melting point of said filler metal in order to form said metal diffusion alloy, causing said wires to be fastened together and resulting in the rigidification of said interlacement, which then forms said rigid surface.
This flexible interlacement may be produced in various ways, for example by knitting, whipping, braiding, lapping or weaving or else by the use of methods for manufacturing nonwoven fibrous products. However, an interlacement in the form of a knit has proved to be particularly advantageous, especially with regard to the diffusion of the heat flux received by said reflective surface.
The electrically conducting wires may consist of a metal core covered with said filler metal. In this case, the surface metal is therefore that of the core. However, as a variant, the electrically conducting wires may consist of a plurality of coaxial layers, at least some of which are made of a materialxe2x80x94which is electrically conducting or possibly insulatingxe2x80x94different from said surface metal.
Among the metals used for producing the electrically conducting wires, mention may be made of metals that are good electrical conductors, such as gold, silver, copper, etc., or else alloys having a low thermal expansion coefficient, such as certain ferro-nickel alloys, or else other metals or metal alloys.
The filler metals are chosen from low-melting-point metals or alloys, such as tin or indium, which are capable of forming a stable alloy with the surface metal by diffusion.
Excellent results have been obtained by choosing copper as the surface metal and indium as the filler metal.
The cross section of the electrically conducting wires may be circular, with a diameter of preferably between 6 and 20 microns, or else flattened, with a thickness of, again, preferably between 6 and 20 microns and a width of preferably between 0.2 and 1.5 mm. In these cases, the thickness of the coating of the filler metal may be between 10 xc3xa5ngstrxc3x6ms and 1 micron.
In order to give said surface a desired thickness, it is preferable to apply uniform pressure to the shaped flexible interlacement during the temperature rise.
The surface according to the present invention may be uniform, with no holes. In this case, a relatively close interlacement is provided and the application of said uniform pressure makes it possible to close off any openings in the interlacement. As a variant, said surface may include holes, provided at the time of producing said interlacement.
In a preferred embodiment, the surface obtained by the rigidified interlacement is reinforced by a reinforcement placed against one of the faces of said interlacement and fastened to said face. Thus, the rigidified interlacement in this case forms only the active reflective part of said surface. Such a reinforcement may have a fiber/cured-matrix composite structure. It is then advantageous for the surface and the reinforcement to be fastened together by adhesive bonding using the resin of said matrix, the reinforcement being formed on said surface. Of course, for this purpose the cure temperature of the resin must be less than the melting temperature of the stable metal diffusion alloy.
Thus it may be seen that, by virtue of the invention, a surface reflecting electromagnetic waves is obtained by diffusion soldering of the electrically conducting wires of the interlacement.
What are obtained by implementing the present invention are, inter alia, antenna reflectors able to operate at frequencies of bets en 18 GHz and more than 45 GHz.